This sounds useful, thanks for building it! In my daily job I have to transfer hundreds of GB each day from/to S3, so this seems like a nice improvement over the standard "aws-cli" from Amazon. Did you have any chance of comparing the two for performance?

My experience seems to show that the concurrent upload manages to saturate our gigabit ethernet connected to our 10 gigabit internet link, which is as good as was-cli manages. I might be able to do some more sensible things if I add better support for how many concurrent uploads occur by splitting the file into length/n sized segments, instead of length/chunkSize minimally sized parts.